The applicants propose to synthesize and study fluorescent probes for hypoxic cells. The optical properties of the new compounds will be evaluated. Their ability to discriminate between hypoxic and oxic cells will be tested in cell cultures. These new probes will selectively bind to biomolecules in hypoxic cells. They will absorb in the 660-670 nm region to match the emission of newly available diode lasers. The long wavelength absorption, high molar extinction coefficients, and high quantum yields of the probe/biomolecule conjugates will allow sensitive detection of hypoxic cells. This will greatly improve the effectiveness of radiotherapy in cancer treatment.